A bellows as a dynamic or movable pressure seal is often employed for sealing a reciprocating plunger or similar device where one end of the bellows is fixed and the other attached to the moving plunger. A bellows is selected because it is leak-proof and a long stroke may be had by simply providing a sufficient number of convolutions. The flexing of the bellows is accomplished by a bending of the bellows material at the convolutions. This bending action is characteristic of all prior types of bellows.
Prior types of bellows intended for such dynamic use are usually made from metal, either stretch formed from a metal tube or brazed-up from metal diaphragms. These bellows are of limited flexural life due to the bending of the metal at the convolutions which generates unpredictable stresses and leads to early fatigue cracking. Metal bellows are thus limited to low frequency operation or occasional use. A further disadvantage with metal bellows is their sensitivity to vibration which can cause premature fatigue cracking.
Another type of bellows is made from rubber, either molded in one piece or made up from separate rubber diaphragms sewn together. However this type of rubber bellows is intended for use as a dust cover and the like and is not suitable for pressure service.
Details of the design and application of both prior types of bellows can be found in the manufacturer's literature. An excellent listing of manufacturers of all types of bellows is given in the current publication of;
The Thomas Register of American Manufacturers PA1 Five Penn Plaza, New York N.Y. 10001 USA PA1 Published annually
There is a need for a long stroke positive pressure seal which can be operated at high cycle rates for extended periods. Up to now the usual solution for high-speed reciprocating service has been to employ a sliding seal such as a rod sealed by packing or pre-formed rings. Sliding seals always permit some leakage and are thus not positive sealing. Also they have the disadvantage of requiring lubrication which in some applications is objectionable.